rendering frame with image save every .... ( 100?) samples
You can render an animation with 100 (same) frames after clicking the variable seed button (clock icon), and setting samples to 100.



Averaging image1 with image2 would be equivalent to the image with 200 samples, averaging images 1, 2 and 3 would be equivalent to the image with 300 samples and so on.

Photoshop or gimp could help you with that but you could do it more quickly with imagemagick

convert 1.jpg 2.jpg 3.jpg ... N.jpg -average result.jpg


Here is the code in bash that echoes the commands to run 

for i in `seq 1 10`; do
    echo -n "convert "
    for j in `seq -f "%03g" 1 $i`;
        do
                echo -n "$j.png "
        done    
    echo "-average result.jpg"
done


and here are the commands for 10 images in case you are on windows

convert 001.png 002.png -average result.jpg
convert 001.png 002.png 003.png -average result.jpg
convert 001.png 002.png 003.png 004.png -average result.jpg
convert 001.png 002.png 003.png 004.png 005.png -average result.jpg
convert 001.png 002.png 003.png 004.png 005.png 006.png -average result.jpg
convert 001.png 002.png 003.png 004.png 005.png 006.png 007.png -average result.jpg
convert 001.png 002.png 003.png 004.png 005.png 006.png 007.png 008.png -average result.jpg
convert 001.png 002.png 003.png 004.png 005.png 006.png 007.png 008.png 009.png -average result.jpg
convert 001.png 002.png 003.png 004.png 005.png 006.png 007.png 008.png 009.png 010.png -average result.jpg